Protective layers for a glass barrier in a photovoltaic device

ABSTRACT

A photovoltaic device includes at least one photovoltaic cell, a flexible glass layer formed over the at least one photovoltaic cell and a transparent and abrasion resistant film which includes an organic-inorganic hybrid material formed over the glass layer.

FIELD OF THE INVENTION

Embodiments described herein relate generally to photovoltaic devicesand modules, and more specifically to flexible photovoltaic devices andmodules comprising protective films, layers and coatings.

BACKGROUND OF THE INVENTION

Copper indium diselenide (CuInSe₂, or CIS) and its higher band gapvariants, such as copper indium gallium diselenide (Cu(In,Ga)Se₂, orCIGS), and any of these compounds with sulfur replacing some of theselenium represent a group of materials, referred to as copper indiumselenide CIS based alloys, have desirable properties for use as theabsorber layer in thin-film solar cells as used in photovoltaic modules.These layers are susceptible to damage from water and/or water vapor.

Photovoltaic (“PV”) modules used in residential structures and roofingmaterials for generating electricity often require additional protectionfrom environmental damage, such as an ingress of water, that can reducean active lifetime of the photovoltaic system. Additionally, thesemodules require protection from hail, rocks, or other objects that mayimpact their surfaces.

Rigid or flexible sheets of glass may be used to support and/or provideprotection to the underlying semiconductor layers. These sheets,however, may themselves be susceptible to cracking when impacted,thereby exposing the semiconductor layers to moisture and otherenvironmental conditions that diminish the lifetime of the cell orcompletely destroy it. Also, certain impacts may cause cracks that donot extend to the underlying semiconductor layers initially, but maypropagate over time, for example during thermal expansion andcontraction cycles resulting from change of temperature during the day,or over several months and seasons.

Additionally, flexible glasses are susceptible to weakness from microscratches produced during processing, and/or abrasion during weathering.These microscratches and abrasions act as stress concentrators and/orcrack initiation sites which may compromise resistance to impact and/orresistance to moisture barrier properties.

Furthermore, plural impacts over a narrow radius can exceed the tensilestrength of the glass and cause breakage.

SUMMARY

One embodiment of this invention provides a photovoltaic device,including at least one photovoltaic cell, a flexible glass layer formedover the at least one photovoltaic cell, and a transparent planarizinghardcoat formed on the glass layer wherein the planarizing hardcoat isin compressive stress and the glass layer is in tension.

Another embodiment provides a method of making a photovoltaic device,including the steps of providing at least one photovoltaic cell, andforming a flexible glass layer having a transparent planarizing hardcoatover the at least one photovoltaic cell such that the planarizinghardcoat is in compressive stress and the glass layer is in tension.

Another embodiment provides a photovoltaic device, including at leastone photovoltaic cell, a flexible glass layer formed over the at leastone photovoltaic cell, and a transparent and abrasion resistant filmcomprising an organic-inorganic hybrid material formed over the glasslayer.

Another embodiment provides a method of making photovoltaic device,including the steps of providing at least one photovoltaic cell, andforming a glass layer over the at least one photovoltaic cell. Atransparent and abrasion resistant film comprising an organic-inorganichybrid material is located over the glass layer.

Another embodiment provides a photovoltaic device, including at leastone photovoltaic cell, a flexible glass layer formed over the at leastone photovoltaic cell. The flexible glass layer has a first majorsurface facing the at least one photovoltaic cell and a second majorsurface facing away from the at least one photovoltaic cell. A firstencapsulant layer is formed over the first major surface of the flexibleglass layer, and has a modulus of less than 100 MPa at room temperature.A second encapsulant layer is formed over the second major surface ofthe flexible glass layer, and comprises a composite material comprisinga polymer matrix containing a filler material.

Another embodiment provides a photovoltaic device, including at leastone photovoltaic cell and a flexible glass layer formed over the atleast one photovoltaic cell. The flexible glass layer has a first majorsurface facing the at least one photovoltaic cell and a second majorsurface facing away from the at least one photovoltaic cell. A firstencapsulant layer is formed over a first major surface of the flexibleglass layer, and has a modulus of less than 100 MPa at room temperature.A second encapsulant layer is formed over at least a middle portion of asecond major surface of the flexible glass layer, and has a thickness ofgreater than 500 μm and a modulus of less than 100 MPa at roomtemperature.

Another embodiment provides a photovoltaic device, including at leastone photovoltaic cell and a flexible glass layer formed over the atleast one photovoltaic cell. The flexible glass layer has a first majorsurface facing the at least one photovoltaic cell and a second majorsurface facing away from the at least one photovoltaic cell. A firstencapsulant layer is formed over a first major surface of the flexibleglass layer, and has a modulus of less than 100 MPa at room temperature.A second encapsulant layer is formed over a second major surface of theflexible glass layer, and has a thickness of less than 500 μm and amodulus of greater than 500 MPa at room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side cross-sectional view of one embodiment of aphotovoltaic device comprising a protective barrier.

FIG. 2 a shows a side cross-sectional view of one embodiment of aphotovoltaic device protective barrier comprising a planarizing hardcoatformed on a flexible glass layer.

FIG. 2 b is a partial side cross-sectional view of the photovoltaicdevice protective barrier of FIG. 2 a with a superimposed stressdistribution.

FIG. 3 shows a side cross-sectional view of another embodiment of aphotovoltaic device protective barrier comprising an abrasion resistantfilm comprising an organic-inorganic hybrid material formed over aflexible glass layer.

FIG. 4 shows a side cross-sectional view of other embodiments of aphotovoltaic device protective barrier comprising a first encapsulantlayer formed over a first major surface of a flexible glass layer, and asecond encapsulant layer formed over a second major surface of theflexible glass layer.

DESCRIPTION OF THE EMBODIMENTS

As used herein, the term “module” includes an assembly of at least two,preferably more than two photovoltaic cells, such as 3-10,000 cells, forexample. The photovoltaic cells of the module can be photovoltaic cellsof any type. Each of the photovoltaic cells of the module can be a CISbased alloy (e.g., CIGS) type photovoltaic cell described above.Preferably, the photovoltaic cells of the module are thin filmphotovoltaic cells. The thin film photovoltaic cells of the module canbe located adjacent to each other such that an interconnect provideselectrical connection between them. An exemplary interconnect isdescribed in U.S. patent application Ser. No. 11/451,616 filed on Jun.13, 2006 and incorporated herein by reference in its entirety.

FIG. 1 illustrates a photovoltaic device 1000. The photovoltaic device1000 in FIG. 1 includes: a) a photovoltaic cell 10 that includes a firsttransparent electrode 1 adapted to face the Sun, a second electrode 2adapted to face away from the Sun and a photovoltaic material 3 disposedbetween the first and the second electrodes, and b) a transparentprotective barrier 100. Electrode 1 may comprise a transparentconductive metal oxide, such as indium tin oxide, zinc oxide, aluminumzinc oxide or a combination thereof. Electrode 2 may comprise a metal ormetal alloy, such as molybdenum or alloys thereof. The photovoltaicmaterial 3 may include a semiconductor p-n or p-i-n junction, such as ap-CIGS absorber and n-CdS layers. The photovoltaic device 1000 mayinclude a substrate 4. In some cases, the substrate 4 can comprise afoil or plate 4 on which an electrode 2 is disposed. In some othercases, the electrode 2 material can be eliminated and the substrate 4can comprise a conductive plate or foil 4, such as a steel foil, whichacts as the second electrode of the cell. Substrate 4 can be a flexiblesubstrate and photovoltaic cell 10 can be a flexible photovoltaic cellwhich can be rolled up into a roll without breaking or becominginoperative.

The transparent protective barrier 100 is disposed over the photovoltaiccell 10 to provide environmental protection and impact protection to thecell. When the photovoltaic cell is part of a photovoltaic module, theprotective barrier can be formed continuously over other photovoltaiccells in the module. The transparent protective barrier 100 preferablycomprises at a thin, coated flexible glass layer. In some cases, theprotective barrier 100 can be a self-supporting, i.e., a free standingglass layer. The self-supporting layer can be in a form of a roll,ribbon, web, foil or a sheet. Any suitable glass material may be usedfor the glass layer, such as soda lime glass, borosilicate glass, lowalkali soda lime glass, etc. The glass layer may be sufficiently thin,such as having a thickness of 50-500 μm, to provide flexibility to theglass layer (e.g., so that the glass layer may be rolled up into aroll).

The protective barrier 100 can include one or more transparent sublayers(not shown in FIG. 1). The term “transparent” includes layers andmaterials which allow at least 75% of visible solar radiation, such as80-100% of this radiation to be transmitted to the cell(s). In somecases, the transparent protective barrier can also include a weatherabletop sheet or layer (not shown in FIG. 1) on the Sun facing side of thebarrier), for protecting the cell(s) from moisture. The top sheet orlayer may be a flouorpolymer layer, such as a ETFE or FEP weatherabletop layer.

The flexible glass layer has one or more inorganic or organic-inorganichybrid protective layers on the surface of the glass layer that facesthe Sun (i.e., on the major surface of the glass layer which faces awayfrom the cell 10). The protective layer(s) may provide one or more ofthe following advantages: they may fill any existing microcracks and/orprevent formation of new ones, they may prevent water contact andinteraction with the glass layer surface or with any defects on theglass layer surface, and/or they may decrease the impulse of impactsand/or increase the impact area when an object (e.g., hail, rocks, treebranches, etc.) impacts the barrier 100.

The photovoltaic device in FIG. 1 can be encapsulated with one or moreencapsulating layers (not shown in FIG. 1) between the cell 10 andbarrier 100 and below the cell 10. The photovoltaic device 1000 can beformed on a structure, such as a building roof, etc., with theprotective barrier 100 formed on the Sun facing side of the photovoltaiccell 10. As noted above, the barrier 100 and cell 10 may be flexible,such that the device 1000 may be rolled up into a roll without breakingor becoming inoperative. Alternatively, the device 1000 may besemi-rigid, meaning that it can be bent without breaking but cannot berolled up into a roll. The photovoltaic device 1000 can manufacturedinto a roll, then be transported to its installation location, beunrolled from the roll and installed over the structure at theinstallation location.

Planarizing Hardcoat

FIG. 2 a illustrates one embodiment 200 of a protective barrier 100 thatcan be formed on at least one photovoltaic cell 10 in FIG. 1. Theprotective barrier 200 can comprise the flexible glass layer 210 formedover at least one photovoltaic cell, for example, the at least one ofphotovoltaic cell 10 of FIG. 1.

In this embodiment, the protective barrier 200 may include a transparentplanarizing hardcoat 220. The transparent planarizing hardcoat can beformed directly on the top surface of the glass layer 210 facing the Sun(i.e., formed on the top surface of layer 210 facing away from the cell10 shown in FIG. 1). The planarizing hardcoat 220 can be in compressivestress and the glass layer 210 can be in tension as depicted in FIG. 2b. In FIG. 2 b, the imaginary vertical dotted line 250 indicates aneutral state between tension and compression. The imaginary dashed line260 indicates the state of the material through which the dashed line ispassing. The position of the dashed line 260 to the left of the dottedline 250 indicates a compressive stress in the material through whichthe dashed line is passing. The position of the dashed line 260 to theright of the dotted line 250 indicates tension in the material throughwhich the dashed line is passing. Therefore, as shown in FIG. 2 b, mostor all of the glass layer 210 is in tension while most or all of thehardcoat 220 is in compression. The hardcoat prevents damage to thesurface of the glass layer and prevents crack propagation by being incompressive stress, fills any existing microcracks in the glass layerand/or prevents or reduces formation of new ones.

The hardcoat 220 can be formed over at least two major opposing surfacesof the glass layer 210. In other words, the hardcoat can be formed on afirst major surface 231 of the glass layer which faces the at least onephotovoltaic cell, for example the at least on photovoltaic cell 10 ofFIG. 1, and on a second major surface 223 of the glass layer 210 whichfaces away from the at least one photovoltaic cell. The planarizinghardcoat may be formed over all surfaces of the glass layer 210 (i.e.,over the major surfaces and the edge surface(s)).

The hardcoat 220 can provide, among other things, impact andenvironmental protection to the glass layer 210 and/or to the at leastone photovoltaic module, device and/or cell.

The hardcoat 220 can have a thickness of 0.1-5.0 μm. The hardcoat 210can be harder than the glass 210, can have the same hardness as theglass, or may have a lower hardness than the glass. Preferably, thehardcoat 220 is harder than the glass layer 210. The hardcoat 220 maycomprise a moisture barrier, for example a dense moisture barrier.

A material comprising the hardcoat 220 can be selected from any suitablematerials, preferably inorganic or hybrid organic-inorganic materials.For example, the hardcoat 220 may comprises silsequioxane, silicon oxideformed from perhyodropolysilazane, aluminum phosphate, silicates, oralumina. Hardcoat 220 can be selected from AQUAMICA® (available fromClariant Corp., Charlotte, N.C.), CERAMABLE organosilicate (availablefrom UpChemical, China), CERABLAK™ (available from Applied Thin Films,Inc., Evanston, Ill.). Hardcoat 220 can be a spin-on type material whichis deposited at a low temperature, such as below the glass 210transition temperature, such as at least 50° C. below the glasstransition temperature.

If desired, the hardcoat 220 may be densified after deposition. Forexample, the hardcoat 220 may be densified by a low temperature anneal.During the optional densification and/or during processing of thephotovoltaic device 1000, the planarizing hardcoat material shrinks andgoes into compressive stress. In other words, the planarizing hardcoatover the glass can perform as a tempered layer. The planarizing hardcoatcan be harder than the glass of glass layer 210 and can be at least asflexible as the glass. The photovoltaic device, with or without theprotective barrier 200 described herein, can be rolled into a roll.

Inorganic/Organic Hybrid Film

FIG. 3 illustrates another embodiment 300 of a protective barrier 100that can be formed on at least one photovoltaic cell, such as thephotovoltaic cell 10 in FIG. 1. The protective layer 300 can comprise aflexible glass layer 310 described above formed over at least onephotovoltaic cell, for example, the at least one of photovoltaic cell 10of FIG. 1.

The protective barrier 300 may also include a transparent and abrasionresistant film 320 comprising an organic-inorganic hybrid materialformed on or over the glass layer 310. The protective barrier 300 mayfurther include a transparent planarizing hardcoat (not shown in FIG.3), for example the hardcoat 220 as described with respect to FIG. 2 a.The transparent planarizing hardcoat 220 may be formed between the glasslayer 310 and the abrasion resistant film 320 such that the hardcoat isin compressive stress while the glass layer 310 is in tension.

The film 320 can be formed over at least one major surface and two minoropposing surfaces of the glass layer 310. In other words, the hardcoatcan be formed on or over major surface 323 of the glass layer 310 whichfaces away from the at least one photovoltaic cell and at least one edgesurface of the glass layer 310. The film 320 may be formed over allsurfaces of the glass layer 310.

The film 320 can comprise an organic matrix formed of organic materialwith either inorganic particles (not visible in FIG. 3) dispersedtherein or inorganic groups grafted thereon. The particles of film 320can comprise discrete particles of substantially the same diameter ordifferent diameters, or fibers of substantially the same lengths or ofdifferent lengths, or combinations of particles and fibers. Since thefilm 320 provides scratch and abrasion resistance, the particles orfibers may have a size or diameter than is the same, smaller than orgreater than the thickness of the film's 320 matrix. In other words, theparticles or fibers make the soft polymer matrix stiffer. However, sincethe polymer matrix itself would not suffer significant damage from ascratch, the particles or fibers do not need to have a smaller size thanthe thickness of the film 320 to provide scratch resistance to the film320. Instead, the film 320 provides scratch resistance to the underlyingglass layer 310 which is prone to crack after being scratched.

For example, the film 320 can comprise a polymer and at least one offumed silica and titanium dioxide particles or fibers. The organicmaterial can comprise a hydrophobic fluoropolymer. The organic materialcan comprise can comprisevinyltriethoxysilane-tetraethoxysilane-polyfunctional acrylate hybridpolymer hard coat, fluorinated ethylene propylene (FEP) with or withoutabrasion resistant additives, ultra-high molecular weight polyethylene(UHMWPE), polyether ether ketone (PEEK), ethylene tetrafluoroethylene(ETFE), polyvinylidene fluoride (PVDF), and/or polyhedral oligomericsilsequioxanes.

The protective layer 300 can provide, among other things, impact andenvironmental protection to the glass layer and/or to the at least onephotovoltaic cell. Therefore, in one embodiment, the film 320 can beweather resistant and/or scratch resistant. The glass layer 310 can havea thickness of 50-500 μm and the film 320 can have a thickness of 1-100μm.

The photovoltaic device, with or without the protective barrier 300described herein, is preferably flexible and can be rolled into a roll.

High Modulus Composite and Low Modulus Encapsulating Layers

FIG. 4 illustrates another embodiment 400 of a protective barrier 100comprising high and/or low modulus layers that can be formed on or overat least one photovoltaic cell such as the photovoltaic cell 10 inFIG. 1. The protective layer 400 can comprise a flexible glass layer 410formed over the at least one photovoltaic cell, for example, the atleast one photovoltaic cell 10 of FIG. 1. The flexible glass layer 410can have a first major surface 431 facing the at least one photovoltaiccell and a second major surface 423 facing away from the at least onephotovoltaic cell (e.g., toward the Sun). Additionally, the photovoltaiccell 10 can comprise a flexible photovoltaic cell formed on a flexiblesubstrate.

Protective barrier 400 may include one or more transparent sublayers,for example a first encapsulant layer 430, a second encapsulant layer422, and an optional weather barrier 424.

The first encapsulant layer 430 can be formed over the first majorsurface 431 of the flexible glass layer 410, and can have a modulus ofless than 100 MPa at room temperature, such as 2-50 MPa. For example,the first encapsulant layer 430 can comprise a polymer or glass layerhaving a modulus of less than 50 MPa at room temperature.

The second encapsulant layer 422 can be formed over the second majorsurface 423 of the flexible glass layer 410, and can comprise acomposite material comprising a polymer matrix containing a fillermaterial. The second encapsulant layer 422 may have a modulus above 100MPa, such as above 500 MPa, for example 100-1000 MPa, including 500 to1000 MPA.

Thus, a softer layer 430 is formed over the bottom, cell facing surfaceof the glass layer 410, and a harder layer 422 is formed over the top,Sun facing surface of the glass layer 410. The softer layer 430 providesa cushion which allows the glass layer 410 to bend or flex during impacton the glass layer 410. The harder layer 422 provides scratch and/orimpact resistance to the glass layer 410.

The filler material can comprise at least one of fibers, scrim,nanotubes, nanowires and particles. For example, the filler material cancomprise organic, inorganic or glass fibers which are weaved withpreferred orientation or matted without preferred orientation. Thefiller material can alternatively comprise transparent particles, suchas SiO₂, TiO₂ or the like. Additionally, the filler material can be of asize which is less than a thickness of the second encapsulant layer 422to provide an impact resistance to the second encapsulant layer.

The polymer matrix can comprise a UV stable polymer having a modulus ofless than 100 MPa at room temperature. Additionally, the filler materialcan increase a modulus of the composite material to at least 100 MPa atroom temperature.

Low Modulus Encapsulating Layers

Alternatively, the first encapsulant layer 430 can have a modulus ofless than 100 MPa at room temperature and the second encapsulant layer422 can have a thickness of greater than 500 μm, and a modulus of lessthan 100 MPa at room temperature. In other words, soft encapsulatinglayers are formed on both sides of the flexible glass layer 410. Theunderlying layer 430 provides a cushion which allows the glass layer 410to bend or flex during impact on the glass layer 410. The thick and softoverlying layer 422 absorbs the impact of the object and spreads theimpact radius to lower the effect of the impact on the glass layer 410.

The first encapsulant layer 430 can comprise a polymer or glass layerhaving a modulus of less than 50 MPa at room temperature, such as 5-50MPa, and the second encapsulant layer 422 can comprise a glass orpolymer layer having a modulus of 5 to 50 MPa at room temperature and athickness of 550 to 5000 μm.

High Modulus Glass/Polymer and Low Modulus Encapsulating Layers

Alternatively, the first encapsulant layer 430 can have a modulus ofless than 100 MPa, such as 5-50 MPa at room temperature, and the secondencapsulant layer 422 can have a thickness of less than 500 μm and amodulus of greater than 500 MPa at room temperature, such as 500-1000MPa.

For example, the first encapsulant layer 430 can comprise a polymer orglass layer having a modulus of less than 50 MPa at room temperature. Anexample of a soft glass suitable for layer 430 is Wacker amorphoussilicon polymer having a modulus of about 10 MPa.

The second encapsulant layer 422 can comprise a hard glass or polymerlayer having a modulus of 500 to 1000 MPa at room temperature and athickness of 50 to 250 μm. An example of a hard glass polymer isSentryGlas® architectural safety glass interlayer made by DuPont.

In the embodiments illustrated in FIG. 4, the encapsulant layers 422,430 may decrease the impulse of impacts and/or increase the impact areawhen an object (e.g., hail, tree branches, etc.) impacts the barrier100.

In the above embodiments, the at least one photovoltaic cell 10 cancomprise a flexible photovoltaic cell formed on a flexible substrate andthe photovoltaic device 1000 is flexible and can be rolled up in a roll.Additionally, in any of the above embodiments, an optional weatherbarrier 424 may be added over the protective or encapsulating layer(s).The weather barrier 424 can comprise a fluorinated polymer weatherbarrier and can be formed over the second encapsulant layer. Forexample, the fluorinated polymer can be ETFE, FEP, or the like.

It is to be understood that the present invention is not limited to theembodiments and the examples described above and illustrated herein, butencompasses any and all variations falling within the scope of theappended claims. For example, as is apparent from the claims andspecification, not all method steps need be performed in the exact orderillustrated or claimed, but rather in any order that allows the properformation of the solar cells described herein.

What is claimed is:
 1. A photovoltaic device, comprising: at least onephotovoltaic cell; a flexible glass layer formed over a top surface ofthe at least one photovoltaic cell; and a transparent and abrasionresistant film comprising an organic-inorganic hybrid material formeddirectly on a first major surface and a second major surface of theglass layer, wherein the first major surface faces away from the atleast one photovoltaic cell and the second major surface faces towardthe at least one photovoltaic cell, such that the glass layer and thephotovoltaic cell are separated by the film; wherein the film is furtherformed directly on at least two minor opposing surfaces of the glasslayer.
 2. The photovoltaic device of claim 1, wherein the film comprisesan organic matrix formed of the organic material with either inorganicparticles dispersed therein or inorganic groups grafted thereon.
 3. Thephotovoltaic device of claim 1, wherein the film is scratch resistantand comprises a polymer and at least one of fused silica and titaniumdioxide.
 4. The photovoltaic device of claim 1, wherein the organicmaterial is a hydrophobic fluoropolymer.
 5. The photovoltaic device ofclaim 1, wherein the organic material comprisesvinyltriethoxysilane-tetraethoxysilane-polyfunctional acrylate hybridpolymer hard coat, FEP with or without abrasion resistant additives,UHMWPE, PEEK, ETFE, PVDF, or polyhedral oligomeric silsequioxanes. 6.The photovoltaic device of claim 1, wherein the film has a thickness of1-100 μm.
 7. The photovoltaic device of claim 1, wherein the glass layerhas a thickness of 50-500 μm
 8. The photovoltaic device of claim 1,wherein the film is weather resistant.
 9. A method of makingphotovoltaic device, comprising: providing at least one photovoltaiccell; and forming a glass layer over a top surface of the at least onephotovoltaic cell; wherein a transparent and abrasion resistant filmcomprising an organic-inorganic hybrid material is located directly on afirst major surface and a second major surface of the glass layer, anddirectly on at least two minor opposing surfaces of the glass layer,wherein the first major surface faces away from the at least onephotovoltaic cell and the second major surface faces toward the at leastone photovoltaic cell, such that the glass layer and the photovoltaiccell are separated by the film.
 10. The method of claim 9, wherein thefilm comprises an organic matrix formed of the organic material witheither inorganic particles dispersed therein or inorganic groups graftedthereon.
 11. The method of claim 9, wherein the film is scratchresistant and comprises a polymer and at least one of fused silica andtitanium dioxide.
 12. The method of claim 9, wherein the organicmaterial is a hydrophobic fluoropolymer.
 13. The method of claim 9,wherein the organic material comprisesvinyltriethoxysilane-tetraethoxysilane-polyfunctional acrylate hybridpolymer hard coat, FEP with or without abrasion resistant additives,UHMWPE or PEEK.
 14. The method of claim 9, wherein the film has athickness of 1-100 μm.
 15. The method of claim 9, wherein the glasslayer has a thickness of 50-500 μm.
 16. The method of claim 9, whereinthe film is weather resistant.
 17. The method of claim 9, furthercomprising rolling the photovoltaic device into a roll.